Halo Gas

Figures 1 and 2 above display the properties (assuming a co-rotating halo) of the lines of sight toward 30 early type stars in the Galactic halo. Figure 1 shows the paths projected onto the Galactocentric radius - Galactic altitude plane, and Figure 2 shows the expected velocities (due to differential Galactic rotation) along the line of sight as a function of altitude.

Figure 3 (below) displays the absorption for a line of sight with b = 1° and a distance of 10 kpc at different Galactic longitudes (the ordinate) for a hypothetical ion. The distribution of the absorbing gas is a combination of a co-rotating uniformly distributed layer with a scale height of 250 pc, and a set of co-rotating spiral arms with Gaussian scale heights of 300 pc.

Research activities:

My major interest in Galactic halo gas has been in modeling its large scale distribution and motion.

In collaborated with Savage, I employed a simplified model of Galactic halo gas and demonstrated that the gas appears to co-rotate with the disk gas up to a Galactic altitude of at least 1 kpc. Above that, it may decouple, with the specific altitude of decoupling depending upon the Galactocentric radius.

In additional work with Savage and Sembach, I developed a somewhat more sophisticated, though still heuristic, model Galaxy to produce interstellar absorption profiles for the halo gas and applied them to specific lines of sight. These results were highly suggestive that the decoupling altitude decreases with decreasing Galactocentric radius.

I have recently begun work in this field again. My goal is to utilize IUE data for several halo stars to in order to invert the problem and determine the density and kinematic behavior of the halo by simultaneously fitting a set of profiles to a parameterized model galaxy. The Chi square of the fits is evaluated and the model galaxy parameters are adjusted until an optimum model is determined. Examples of the improvements which can result from this approach are given below.

I am also currently involved in an HST project (PI, Cardelli) which utilizes Galactic rotation to determine whether a Galactocentric abundance gradient can be detected. Analysis of these observations is currently underway.

Some relevant publications:

``Ultraviolet Absorption Toward HD 156359, a Halo Star at 11 Kpc in the Direction l = 329° and b = -15°'', K. Sembach, B.D. Savage, & D. Massa 1991, ApJ, 372, 81

``Ultraviolet Interstellar Absorption Toward HD 163522, A Star at 9 Kpc in the Direction l = 350° and b = -9°'', B.D. Savage, D. Massa & K. Sembach 1990, ApJ, 355, 114

``Ionized Interstellar Gas Located in the Disk and Halo'', B.D. Savage & D. Massa 1987, ApJ, 314, 380

``Absorption by Highly Ionized Gas Near the Galactic Center'', B.D. Savage & D. Massa 1985, ApJ, 295, L9

Derck Massa,
Raytheon ITSS